One problem with prior art electronic ballasts is that the open circuit voltage of an instant start ballast needs to be controlled when there is not a lamp coupled to the ballast. Unfortunately, prior art methods of providing this open circuit voltage control cause substantial variations in the open circuit voltage when used in conjunction with different lengths of cable, or require a high value resonant capacitor which results in a high circulating current. A high circulating current is undesirable in that it increases the conduction losses in the ballast and may result in capacitive mode switching occurring during the striking transients. Therefore, an improved method and apparatus for controlling the open circuit voltage of a high input voltage electronic ballast is needed.
In some prior art ballasts, the voltage on a lamp voltage sensing resistor is used to control the open circuit voltage during striking when no lamp is connected. To accomplish this, the pulse width of one switch of the half bridge is typically controlled. Controlling the pulse width controls the open circuit voltage indirectly by using inductor current to control the voltage on the capacitor. As a result, large open circuit voltage variations often result when external connections to the fixture, such as a connecting cable, add extra capacitance. In ballast implementations that can afford to use a large resonant capacitor and a small inductor, the open circuit voltage variation problem is generally not significant. However, potentially damaging hard switching or capacitive mode switching is often observed with this type of prior art open circuit voltage controlled ballast. Furthermore, the use of a large resonant capacitor makes the resonant tank difficult to design. As a result, these types of ballasts suffer from more conduction losses and/or hard switching during the striking of the lamp than do typical ballasts. Conduction losses and hard switching are undesirable in that they may cause the ballast to fail. A large resonant capacitor, with a striking voltage of two lamps across it, stores a substantial amount of energy. When the striking attempt occurs when there is no load, the striking energy is transferred to the resonant inductor and can saturate the inductor. The result is undesirable hard switching occurring during the striking. Even though a MOSFET can survive the high stress transients in ballasts with a 460V bulk voltage, hard switching is undesirable and should be avoided if possible. Furthermore, for some types of ballasts, it is critically important to avoid hard switching due to their particular susceptibility to damage from transients. Thus, in many of the prior art ballasts, the resonant capacitor value is minimized and a cable compensation circuit is utilized to control the open circuit voltage such that it is substantially constant when coupled to various lengths of cable having various amounts of capacitance. However, these circuits are often overly complex and, thus, decrease the efficiency of the ballast while increasing its cost. Therefore, an improved method and apparatus for controlling the open circuit voltage of a ballast and compensating for the effects any attached cables is needed.